Technical Field
The present invention relates to a flexible display device, and more particularly to a flexible display device having a bending sensing device configured such that an external resistor is mounted in the flexible display device together with a bending sensor, whereby it is possible to lower an output offset voltage, to reduce resistance deviation depending on temperature change, and to sense twisting of the flexible display device.
Description of the Related Art
Display technology for processing and displaying a large amount of information has rapidly grown. In addition, various kinds of display devices have been developed.
Examples of display devices include a liquid crystal display (LCD) device, a plasma display panel (PDP) device, a field emission display (FED) device, and an electroluminescent display (ELD) device. The thickness, weight, and power consumption of the display devices have been continuously reduced. However, it is difficult to manufacture the above-mentioned display devices such that the display devices are thin and flexible, since the display devices use a glass substrate, which withstands high temperatures generated in a manufacturing process.
For this reason, a flexible display device manufactured using a flexible material, such as a plastic film, which is foldable or unfoldable, in place of the conventional glass substrate, which has no flexibility, such that the flexible display device can be curved like paper while maintaining display performance has attracted attention as a next-generation flat panel display device in recent years. The flexible display device has advantages in that the flexible display device is thin, lightweight, impact resistance, and can be curved or bent so as to be folded or rolled for carrying. In addition, the flexible display device may be manufactured to have various forms. Consequently, future applicability of the flexible display device may be extended.
Flexible display devices have completed the testing phase, and mass-production of flexible display devices is imminent. It is expected that the flexible display device provides a new input and output interface different from electronic devices having conventional rigid displays, and it is also expected that newer user experiences may be provided through the new input and output interface.
In recent years, a device for sensing the shape of the flexible display device, configured such that a plurality of bending sensors is disposed at the edge of the flexible display device in order to sense the shape of the flexible display device, has been proposed (see Korean Patent Application Publication No. 10-2014-0132569).
FIG. 1 is a view showing a conventional flexible display device in which a plurality of bending sensors is disposed, and FIG. 2 is a view showing the construction of a conventional measurement unit. FIGS. 3A and 3B are views showing strain gauge circuits, and FIG. 4 is a block diagram showing the detailed construction of a microprocessor (MCU) of FIG. 2.
Referring to FIG. 1, bending sensors 101 and 102 for sensing bending of a flexible display device 100 are arranged along the edge of the flexible display device 100 at predetermined intervals.
Each of the bending sensors 101 and 102 may be a strain gauge. The strain gauge has a characteristic in that the resistance between terminals varies depending on physical tension (elongation) and compression (contraction). In order to sense the shape of the flexible display device 100 using the sensors, it is necessary to provide a measurement unit for signal processing. The measurement unit may be realized as shown in FIG. 2.
The conventional measurement unit may include a bridge circuit 210, an amplifier 220, and an analog to digital converter (ADC) 230.
The bridge circuit 210 is realized by a Wheatstone bridge, which includes one or more strain gauges. Since the resistance variation of each of the strain gauges is very small, the Wheatstone bridge is configured, as shown in FIG. 2, in order to convert resistance variation into voltage variation, which is amplified by the amplifier 220.
Meanwhile, the Wheatstone bridge may use a quarter-bridge circuit for sensing variation of a single strain gauge, as shown in FIG. 3A, or a half-bridge circuit for sensing variation of a pair of strain gauges, one of which is tensioned and the other of which is compressed, as shown in FIG. 3B. That is, in the case in which strain gauges are mounted to opposite surfaces of the flexible display device 100 at the positions at which the bending sensors 101 and 102 are disposed, the strain gauges may sense tensile strain and compressive strain. Consequently, the sensitivities of the sensors are improved.
Meanwhile, in the case in which the bridge circuit 210 of FIG. 2 is configured as a quarter-bridge circuit 320a shown in FIG. 3A, the quarter-bridge circuit 320a may be constituted by R1, R2, R3, and one strain gauge 330a. When power from a power source 310 is distributed to the respective resistors, the amplitude of voltage output from the bridge circuit varies depending on the resistance variation of the strain gauge 330a. 
On the other hand, in the case in which the bridge circuit 210 of FIG. 2 is configured as a half-bridge circuit 320b shown in FIG. 3B, the half-bridge circuit 320b may be constituted by R1, R3, and two strain gauges 330b and 330c. When power from a power source 310 is distributed to the respective resistors, the amplitude of voltage output from the bridge circuit varies depending on the resistance variations of the strain gauges 330b and 330c. The shape of the flexible display device is sensed based on the value of the voltage output from the bridge circuit.
The voltage output from the bridge circuit 210 is input to the amplifier 220, by which a small value of voltage is amplified into a large value of voltage. The amplified voltage is input to the analog to digital converter 230. The analog to digital converter 230 converts an analog signal into a digital signal, which is output to a microprocessor 240. The microprocessor (MCU) 240 determines the shape of the flexible display device 100 based on the values sensed by the sensors.
The detailed construction of the microprocessor 240 is shown in FIG. 4.
That is, the microprocessor 240 includes an input 401, a noise filter 402, a channel compensator 403, a curve point detector 404, a gain controller 405, a bending line detector 406, a slope compensator 407, and a feature extractor 408.
The input 401 receives the digital signal from the analog to digital converter 230. The noise filter 402 filters the change of a sensor value (e.g., provided at the input 401) due to factors other than the bending of the flexible display device 100 from a meaningful signal.
The channel compensator 403 compensates for the deviation between the sensors disposed at the flexible display device 100. In addition, the channel compensator 403 may compensate for the deviation between different sensors used in flexible display devices 100.
The curve point detector 404 analyzes values (for example, voltage values) sensed by the sensors 101 and 102 arranged in a line along each side (i.e. each edge) of the flexible display device 100 to extract the position and feature of a curve point formed at each edge (i.e. each outer region) of the flexible display device 100.
Upon determining based on the value output from the curve point detector 404 that the values output from the sensors 101 and 102 are less than a predetermined reference value or deviate from an input range of the analog to digital converter 230, and therefore it is necessary to control the gain of the amplifier 220 (for example, a variable gain amplifier), the gain controller 405 generates and provides an appropriate gain control signal to the amplifier 220.
Meanwhile, information about curve points detected from outer regions 110, 111, 112, and 113 by the curve point detector 404 is input to the bending line detector 406 in order to be used to determine the shape of the flexible display device 100.
The slope compensator 407 compensates for information about bending of bending lines based on information about slope of the bending lines.
The feature extractor 408 extracts and transmits the position, slope, angle, thickness, and direction of the detected bending lines to an upper layer.
However, the device and method for sensing the bending of the flexible display device have the following problems.
First, in the conventional flexible display device, the bending sensors are attached to the surface of the flexible display device. As a result, it is not possible to accurately sense overall deformation of the flexible display device. In addition, it is not possible to accurately measure stress and strain in layers of the flexible display device and to estimate deterioration of the flexible display device over time and depending on environments.
The reason for this is that the thickness of the conventional strain gauges is about 75 μm (in case of commercial products) and a special bonding agent having a thickness of several tens of μm in order to bond the strain gauges to the flexible display device 100 (e.g., to a surface of the flexible display device 100). In the case in which the thickness of the strain gauges is reduced like the flexible display device, the elasticity of the strain gauges, rather than the elasticity of the flexible display device 100, is critical, with the result that it is not possible to accurately sense the deformation of the flexible display device 100.
In addition, the special bonding agent, provided between the flexible display device 100 and the strain gauges, has a different coefficient of elasticity than the flexible display device 100 (e.g., a surface of the flexible display device 100). In addition, the special bonding agent has viscosity in addition to elasticity. Even when the flexible display device 100 is linearly deformed, therefore, the values measured by the strain gauges are not linear.
Second, in the case in which the bridge circuit is configured as the quarter-bridge circuit shown in FIG. 3A, the bending sensors (strain gauges) are disposed at the edge of the flexible display device, and the other resistors R1, R2, and R3 are formed in a circuit board. As a result, offset and tolerance are generated due to the difference in wiring length between the bending sensors (strain gauges) and the other resistors R1, R2, and R3.
If the signal is amplified in the state in which the offset value is present, the amplified signal exceeds the input margin of the analog to digital converter, with the result that the digitized value becomes saturated.
Third, in the case in which the bridge circuit is configured as the half-bridge circuit shown in FIG. 3B, two bending sensors (two strain gauges) must be in states of being tensioned and compressed. In this case, design of the system may be restricted.
Fourth, in the conventional flexible display device, the bending sensors are attached to the surface of the flexible display device. As a result, it is necessary to form a routing line for transmitting a signal output from each bending sensor through an additional process, or an additional flexible printed circuit (FPC) is required, whereby production costs are increased.
Fifth, in the conventional flexible display device, the bending region has higher flexibility than the remaining region, with the result that the flexible display device may be twisted. In the case in which the flexible display device is twisted, however, it may be incorrectly detected that the flexible display device is bent.